If Napoli doesn't have a second striker that can fill in Lukaku's space, I agree with you that Napoli could struggle to win their matches, but if Napoli has a backup striker they can still make use of until Lukaku returns from injury, the striker can help Napoli win their matches this season. If Napoli feels they need a new striker since Lukaku is injured, the transfer window is still open for them to bring in another striker to start their Serie A campaign.
It seems Napoli should act quickly to consider this before the new season begins, because if Lukaku's injury still hasn't recovered by the start of the season, finding a new striker would certainly be a wise move for Napoli. However, they also need to ensure that the new striker is good, even if not quite on par with Lukaku's achievements. If the new striker is just a mediocre player, it certainly won't be enough to help Napoli start the new season and Napoli could struggle to win consistently this coming season.
Yes, if the situation is like that Napoli should indeed bring in a striker who is great enough for the upcoming season. Because as we know in Serie A or in other competitions that Napoli will participate in, it is certain that the competition will be very tight. So with this, Napoli should consider looking for a striker who can replace Lukaku when he is injured. Because it cannot be denied that Lukaku is quite old at this time. So it is certain that with this Lukaku will definitely be more susceptible to injury.
So with this I think Napoli really have to move faster at this time, if they want to bring a new striker into the team. Because if they buy a new striker when the season really starts, I don't think it's the right idea. Because of course new players need adaptation and of course it takes a short time. In addition, Napoli actually also borrowed a striker this season, but I'm not too sure that the player can play well enough at Napoli. Then Napoli also actually has 2 other strikers in their squad, the players are Cheddira and Ambrosino. I think those two players are also really not fully reliable at Napoli, so with this, Napoli seems to have to buy a new striker at this time.